1. Field of the Invention
The present invention relates to a sensor assembly and a drive unit for a bicycle, and to a bicycle.
2. Description of the Related Art
A sensor assembly which can detect a wheel speed of a wheel is known from JP-UM-56-130163 and the like. This sensor assembly includes a magnet that is attached to a wheel to rotate together with the wheel and a sensor that is fixed to a body frame so that the sensor detects a rotation of the magnet.
In the above sensor assembly, a sensor and a magnet are spaced apart from each other in the direction of a wheel axis. In the event that a wheel is supported so as to rotate smoothly by a body frame such as a front fork, a certain gap distance is provided between the body frame and a wheel in relation to the direction of the wheel axis of the wheel. This gap allows the wheel to rotate smoothly relative to the body frame. In consideration of the accuracy with which the wheel is assembled to the body frame, the axial gap cannot be reduced needlessly. This results in a situation in which the sensor fixed to the body frame is spaced apart from the magnet attached to the wheel in the axial direction. In addition, when the distance between the magnet and the sensor is increased, the magnetic force reaching the sensor is attenuated, and therefore, a large magnet must be used.